Intercommunicating system



July 18, 1950 A. MONTANI mmacomumcnmc SYSTEM Filed Sept. 24, 1945 mmEImsZ 43 02:.

INVENTOR ANGELO MONTANI ATT'ORNEY UNITED: STATE Patented July 18, 1950 s PATENT OFFICE 8 Glaims'. The present invention relates in 'genral to inter'corzimunicating systems, and in particular to an improved arrangement for preventing feed back or singing in such systems:

Numerous methods have been proposed for eliminating feedback wasp-easing intercommuhicating systems, Gne of the most commonly used methods involves the provision of talk-'- listei'PkeyS at each station, which keys are a l-- ternately operated to successively condition the 1 syste'rri for transmission in opposite directions.

principal objections to the use of talk- Iis'ten keys is that they'prevent free use ofttl-i e speakeifs hands for taking notes or searching through records during-'- a conversation, and that the conversation is unnatural in that the listener is generally unable to interrupt the speaker.

It is accordingly an object of the present in ve'nu'en teobviate the use er talk-listen keyfs by providing means for" rendering the transmitting means and the receiving riieans at ach'station alternately efiective a'ta low frequency rate so asto successively condition the system for transmission in opposite directions. I A feature" of the invention is the provision or a c'forhrnonlow frequency oscillator for renderiii-g the-transmitting and receiving: eit each station alternately effective", me on duency current produced by said oscillator be: ing transmitted from" one station to the" other over'a'speecn signal channel. N The invention may best be understooclironi the" following description taken in corn ion with the" accompanying drawing which senemet: many illustrates an intereonunumeetme System embodying the invention. 'Brijefi'y described, the invention comprises the provision, at each station of an intercoinrnnni eating system, of thermionic tubes which are associated with the transmitting and rece'iving means thereat'so' asto rend-er then'i effective se le'ctively.- A low frequency oscillator is-provid- 'ed atone station for controlling saidthnnionic tubes, a portion of the output of said oscillator being transmitted to the other station to" con;- tiol the' therrriionic tubes thereat over a signal channel which is also used to transmit spe signals between the stations in eithernirectlon; Referring now to the drawing, there are-shown two stations, A and B, which are interconnected by a two conductor line I. tw'o way ariiplifier 2} which may be a Type 21 repeater, is inserted in" line I; Station A includes a transrriitteror niicrophone 3 an associated" thermionic une 4'; a-"i'ecivef 1 nuaspeekec an associated t 2 thermionic tube 6, batteries 1 and i2, transformers and yfilt'ers Ill and H, and a low frequeric'y" oscillator 23. Station B is similarly equipped with the exception that no oscillator is provided thereat. V In the absence of the thermionic tubes l and 6, and the corresponding tubes M and W at stati'on B, singing will usually occur in such a system when there is acoustic coupling between the transmitters and receivers at each station. Thus a signal produced by transmitter 3 will be impressed on the? le'ften'd of line i through tra nsfor'r'ner '8 and filter l0 and will appear in" amplifled form at the right end of line I. The amplinee signa'lis impressed on receiver [5 through filter 20 and transformer It. When the ampli' fied signal is reproduced by the receiver i5 a cor-'- responding signal will be produced by transmit ter 13 due to the acoustic coupling existing therebetween. The signal produced in transmitter l3 will be amplified by the amplifier 2 and" the amplified signal will be; reproduced by receiver 5". Due to the acoustic coupling between receiver 5 and transmitter 3, thesound waves pro duced by: receiver 5 will produce a new signal in transmitter 3' which may reinforce the orig1- n'al signal and; resultin sustained oscillations if, the net gain around the loop is more than the net loss. The frequency at which such" oscillatrons or howling occurs" determined. by the ength of the acoustic paths between the transmitters and receivers and the electrical pha e shift: around the loop. It may be noted th'at'a Similar condition will occur when the transmit: ter" of each station is coupled to the receiver of the other station over an individual signal channel.

The above'des'cribec. difficulty is overcome when talk-listen keys are provided since the trans mitter and receiver at eitherstation are never rendered} effective simultaneously. Applicant has discovered thatif the transmitter 3 and receiver l5 are intermittently rendered effective at'a} low frequency rate, say thirty times per second, and transmitter t3 and receiver 5 are intermittently ,rendered effective during the in tervals when transr'nittelf ii and receiver I 5 are ineffective, normal conversation can be carried (an between stations A and B". In order to avoid the production of parasitic f'reqnencies the trans rfii tters arid receivers must be enabled and? disabled gradually ratherthan instantaneously; 'TIhej use of mechanical switching devices is therefore impractical-. By employingthermionic fllbcs Witli Suitable potential-s applied to the con normally has a nominal anode impedance.

- ing of transformer l9.

trol electrodes thereof, a smooth yet rapid enabling and disabling of the transmitters and receivers is effected. Thus at station A, the thermionic tube 4 is connected in series with transmitter 3 and transformer 8 to battery I. If the transmitter 3 is of the carbon type, the battery 1 serves the dual function of furnishing a source of anode potential for tube 4 and of supplying energizing current to the transmitter 3. Thermionic tube 6 is connected in series with receiver and transformer 8 to battery 1. The control electrodes of tubes 4 and 6 are connected to opposite ends of the secondary winding of transformer 9. The midpoint of this secondary winding is connected to the cathodes of tubes 4 and 6 through a biasing battery l2. A low frequency oscillator 23 is connected to the primary winding of transformer 9. The biasing potential applied to the control electrodes of tubes 4 and 6 by battery l2 has a value such that each tube The potentials induced in the secondary winding of transformer 9 by oscillator 23, which potentials are impressed on the control electrodes of tubes 4 and 6 180 out of phase, causes the anode impedances of these tubes to alternately increase and decrease at a low frequency rate. When the anode impedance of tube 4 is low the transmitter 3 is rendered effective to impress speech signals on line I corresponding to sound waves impressed on the transmitter. During the time that the anode impedance of tube 4 is low, the anode impedance of tube 6 is high, thereby rendering the receiver 5 ineffective to reproduce signals, either incoming from line I or produced by transmitter 3, which appear thereacross in series with tube 6. During a succeeding halfcycle of the voltage wave produced by oscillator 23, the anode impedance of tube 4 is high thereby rendering transmitter 3 ineffective. At the same time the anode impedance of tube 6 is reduced to a low value to render receiver 5 eifective to reproduce signals appearing across the terminals of transformer 8. A portion of the output of oscillator 23 is transmitted through the low pass filter ll, over the left portion of line I, through amplifier 2, over the right portion of line I, and through low pass filter 2| to the primary wind- The secondary winding of transformer 19 is connected to the control electrodes of tubes l4 and 16 so that tubes 4 and i6 simultaneously render transmitter 3 and receiver l5 effective, and tubes 6 and I4 simultaneously render receiver 5 and transmitter l3 effective during the intervals when transmitter 3 and receiver I5 are ineffective. The high pass filters I0 and 20 and the low pass filters H and 2| are provided to isolate the speech waves and the low frequency control wave at each station. This is possible because the lowest frequency signal which must be transmitted for satisfactory reproduction of speech is of the order of 150 cycles per second.

The use of a low frequency for rendering the system alternately efiective to transmit signals in opposite directions is preferable to a frequency above the audible range because of the difiiculty in maintaining synchronism between the keying tubes at the two stations due toincreasing phase shift in the signal channel, amplifier, and filter networks, with increasing frequency. If a separate conductor is employed between the two stations to transmit the control wave derived from oscillator 23 to station B, the difiiculties produced by phaseshift inthe filter networks and amplifier may be circumvented, thereby enabling the use of a control wave having a frequency above the audible range.

While a particular embodiment of the invention has been illustrated, it should be understood that numerous modifications may be incorporated therein without departing from the true spirit and scope of the invention as defined in the suboined claims.

What is claimed is:

1. In an intercommunicating system, two stations, a two-way signal channel between said stations, voice transmitting and voice receiving means at each of said stations coupled to said channel, a pair of vacuum tubes at each of said stations for alternately rendering effective the transmitting and receiving means thereat, a source of alternating current continuously connected to said channel, and circuit arrangements for impressing control voltages derived from said source on the grid electrodes of all of said tubes so as to successively at a predetermined rate condition the system for voice transmission in opposite directions between said stations over said two-way channel.

2. An intercommunicating system as claimed in claim 1 wherein the frequency of said source is less than that of the lowest frequency component in the voice signals transmitted between said stations.

3. An intercommunicating system as claimed in claim 1 wherein said source of alternating current is located at one of said stations, and said circuit arrangements include provisions for transmitting said control voltage over said signal channel to the other of said stations.

4. In an intercommunicating system, two stations, voice transmitting means and voice receiving means at each of said stations, conductors extending between said stations to enable two-way voice communication therebetween, a low frequency source of alternating current at one of said stations, circuit arrangements for continuously transmitting current from said low frequency source to the other station over said conductors, filters individually associated with each of said stations for isolating said low frequency source from said voice transmitting means and said voice receiving means, and means at each of said stations controlled by voltages derived from said low frequency source for successively at a predetermined rate conditioning the system for voice transmission over said conductors in opposite directions between said stations.

5. In an intercommunicating system, two stations, a line extending between said stations atwo-way amplifier connected in said line between said stations, a transmitter and a receiver at each of said stations, a pair of vacuum tubes at each of said stations for selectively coupling the transmitter and receiver thereat to said line, a source of alternating current continuously connected to said line at one of said stations and to said line at the other station through said amplifier, and means for impressing control voltages derived from said source on the grid electrodes of all of said tubes so as to successively at a predetermined rate condition the system for voice transmission in opposite directions between said stations.

6. In a telephone system, a station including a transmitter and a receiver, a line extending from said station over which two-way voice communication with another station is effected, a pair of thermionic tubes, one of said tubes being assoclated with said transmitter so as to control the effectiveness thereof, the other of said tubes being associated with said receiver so as to control the effectiveness thereof, and means comprising a source of alternating current continuously coupled to said tubes for impressing alternating control voltages on the grid electrodes of said tubes in phase opposition corresponding to the frequency of said source, whereby said transmitter and receiver are alternately rendered effective to transmit voice signals over said line and to reproduce voice signals received over said line, respectively.

7. A telephone system as claimed in claim 6 wherein the frequency rate of said alternating control voltages is less than that of the lowest frequency component in the voice signals transmitted from said station.

8. In a telephone system as claimed in claim 6 wherein said source of alternating current is located at one of said stations, and the system includes circuit arrangements for continuously transmitting alternating current from said source over said line to the other said station.

ANGELO MONTANI.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

